Gitlab Upgrade Broke User Email

## Issue

Upgrading Gitlab from 11.7.0 -> 11.7.3 broke some of our jobs. Specifically, `$gitlabUserEmail` isn't being set anymore

### Context
- **Gitlab plugin version**:
- **Gitlab version**: 11.7.3
- **Jenkins version**: 2.150.2
- **Job type**: Pipeline

### Logs & Traces

It looks like the failure is because the user_email in the json payload from gitlab is an empty string.

Also, this problem doesn't occur when using the test button from gitlab.

### Problem description

The gitlab user email environment variable suddenly disappeared with the 11.7.3 update. This worked on 11.7.0.

```
echo """
$gitlabBranch
$gitlabSourceBranch
$gitlabActionType
$gitlabUserName
$gitlabUserEmail
$gitlabSourceRepoHomepage
$gitlabSourceRepoName
$gitlabSourceNamespace
$gitlabSourceRepoURL
$gitlabSourceRepoSshUrl
$gitlabSourceRepoHttpUrl
$gitlabBefore
$gitlabAfter
"""
```

That code is broken on 11.73. I had to remove the `$gitlabUserEmail` from the echo statement (and nothing else) to get the job to work again.